absorption lines: Dark lines in a continious spectrum. Produced when light passes through a gas, where the gas atoms/molecules absorb light at certain frequencies, preventing those frequencies from reaching the observer.
angular velocity: the rate at which the direction of an object is changing.
arcminute (arcmin or '): 1/60 of a degree of angle.
arcsecond (arcsec or "): 1/3600 of a degree of angle.
Cosmic Distance Ladder: The collective expression for different methods of determining distances to astronomical objects. Each method is useful over a certain range of distances and usually is calibrated from the results of another method, hence the expression "ladder".
Doppler Effect: The change of frequency of sound or light due to motion. For example, the whistle of a train drops in pitch as it goes by.
EVA: "Extravehicular activity", also known as "spacewalk". Any activity in space by an astronaut that takes place outside spacecraft.
galaxy: A big group of stars, usually containing many billions of them.
Galaxy: Our galaxy, also called the Milky Way. Its most visible component is shaped like a disk 10000 ly across, and is visible at night as a band across the sky. It contains roughly 10^11 stars.
geosynchronous (or geostationary) orbit: The Earth orbit with 24-hr period (about 40 000 km high). Since satellites in that orbit rotate around Earth in the same time as Earth revolves around itself, they look stationary from Earth. Used for communication satellites.
infrared: The electromagnetic waves with somewhat lower frequencies (longer wavelengths) than visible light. Emitted by objects somewhat cooler than Sun.
luminosity: The power radiated by a star.
open cluster: A relatively loose cluster (small group) of stars, containing a few dozen to a few thousand stars. Found in the disk of the Galaxy.
parallax: The change of an object's apparent direction when viewed from two different points. For example we can judge the distance to nearby (a few meters or closer) objects, because we have two eyes, each looking at the object from a slightly different angle. The line between the two points (here eyes, usually chosen perpendicular to the line of sight) is called the baseline. (In the context of this site usually meant to be due to the orbital motion of Earth. Click for an illustration)
parsec (pc): A distance unit popular among astronomical observers: When you are so far away from Sun that 1 AU spans one arcsec, you are 1 parsec away. It turns out that 1 pc=3.26 ly.
plasma: A gas consisting of charged particles; for example, a gas hot enough that most molecules are ionized.
radial velocity: The component of velocity along the line of sight, i.e the speed with which the object's distance is changing (Click). Can be measured by the Doppler Effect.
rendezvous: Literally, "meeting". Here, the meting and joining ("docking") of two spacecraft in space.
RTG (radioisotope thermal generator): The power source used in most space probes sent to the outer solar system, e.g. the Pioneers, Voyagers, Galileo, Cassini, etc. It contains some radioactive material and a "thermocouple" that creates electricity from the temperature difference between the warm radioactive material and cold space. It is needed where there is not enough sunlight for solar panels; and is reliable for tens of years because it has no moving parts.
spacewalk: See EVA
spectrum: The distribution in frequency (or wavelength) of electromagnetic energy emitted by an object. When the object emits mostly visible light, the spectrum may show visually each color and its brightness.
tangential velocity: The component of velocity perpendicular the line of sight (Click).
thermal radiation: The radiaton given off by an object due to its temperature. The spectrum is characteristic, with the peak frequency proportional to temperature (Wien's Law). So, approx. 10-1000 K objects radiate most strongly in infrared, 1000 - 10 000 K objects in visible light, 10 000 - 100 000 K objects in ultraviolet.
ultraviolet: The electromagnetic waves with somewhat higher frequencies (shorter wavelengths) than visible light. Emitted by objects somewhat hotter than Sun.
von Neumann machine: A hypotethical self-replicating probe with --possibly limited-- artificial intelligence. It would land on a planet, dig the soil, build factories to extract elements and make many copies of itself, which would then be launched in search of other planets.
zij: Medieval islamic name for an astronomical treatise that usually contains tables for calculating the positions of the Sun, Moon, and planets. It might also contain a star catalogue (For example, that compiled by Ulugh Beg).